engine shaking

i have a ford escort 1997. a few months ago i replace the motor mount on the vechile. but today i was driving with no a/c and began to shake very bad. when i turned on the a/c the engine began to run smooth with no shaking or jerking. what could be the problem.:banghead::shakehead:frown::frown::frown:: shakehead:banghead::banghead::banghead::banghead: